1. Field of the Invention
The present invention relates to a method of and an apparatus for estimating the shape of a human auditory filter.
2. Description of the Relevant Art
Hearing characteristic tests for the hearing-impaired that are most frequently conducted nowadays are a hearing test (measurement of audiogram) and a speech intelligibility test. In the hearing test, it is possible to ascertain frequency characteristics of the threshold of hearing of the hearing-impaired. In the speech intelligibility test, it is possible to determine intelligibility or hearing ability of the hearing-impaired.
However, the hearing characteristics of the hearing-impaired vary with individuals. It is therefore considered that these two methods can only grasp a part of the complicated hearing characteristics.
It is generally said that the hearing-impaired have the deterioration of their frequency selectivity in addition to the deterioration of their hearing ability and the deterioration of their speech intelligibility. The frequency selectivity is the capability of distinguishing two sounds that are different in frequency. Normal hearing can distinguish two different sounds that are close in frequency, for example, 1 kHz v. 1.2 kHz, but the hearing-impaired whose frequency selectivity has deteriorated cannot distinguish these two different sounds.
When the extent of deterioration in the frequency discrimination becomes greater, it leads to deterioration of ability of discrimination of speech or deterioration of ability of discrimination of speech sound under noisy conditions. Accordingly, knowing the extent of deterioration in the frequency discrimination is very useful for a diagnosis of hearing impairments, a grasp of the hearing characteristics of the hearing-impaired, fitting of a hearing aid, etc.
In recent years, an auditory filter has been suggested as a model for expressing a mechanism of frequency analysis of the human sense of hearing. This is a method for expressing a mechanism of frequency analysis of the human inner ear by band-pass filter banks. The shape of each filter (i.e. an auditory filter) within these filter banks is usually measured using notched noise masking. A simplified method for measuring the frequency discrimination for the hearing-impaired that uses a theory of this auditory filter is disclosed in Japanese Unexamined Patent Publication No. HEI 6-327654 (1994). It is known that the shape of a human auditory filter can be modeled by a roex (p, r) filter.
In the measurement of the auditory filter by the notched noise masking, it is said that the shape of each subject's auditory filter can be measured with high accuracy. However, this measurement takes a long time and it is practically impossible for clinicians or audiologists to measure the shape of the auditory filter of each of the hearing-impaired at any time. Japanese Unexamined Patent Publication No. HEI 6-327654 (1994) suggests a method of measuring whether or not the frequency discrimination has deteriorated in a short time. According to this method, it is possible to judge whether there is any deterioration in the frequency discrimination, but is not possible to measure the extent of deterioration in the frequency discrimination, or the shape itself of the auditory filter.
Further, it is said that the shape of the auditory filter varies with the level of the input signal. It is therefore said that a masker (i.e., notched noise) whose level is about 40 dB/Hz is most suitable for the measurement of the auditory filter of people of normal hearing judging from past information. It is also said that when measured at a level higher than 40 dB/Hz, the shape of the auditory filter becomes broad. However, such a shape change in the auditory filter as seen in people of normal hearing does not always occur in all the hearing-impaired according to these levels. There is no method of measuring the shape change characteristics of the auditory filter according to each level of hearing-impairment to date.